This invention relates generally to hydraulic apparatus for controlling fluid flow to a hydraulic motor. In particular, it relates to a hydraulic system for controlling fluid flow to a rotary motor which drives a cooling fan associated with a vehicle cooling system, and to a pilot-operated pressure-compensated valve which is used in the system.
The cooling fan for a vehicle engine has been driven mechanically from the engine crankshaft, either directly or through a friction clutch. Automotive engineers have recognized for some time that there are problems involved with driving cooling fans in such a manner. In particular the problems involve excessive use of fuel and excessive noise. One attempt at solving these problems has been through the use of viscous fluid drives for driving the fan. However, viscous fluid drives also have presented significant engineering problems. Fan drive systems and the problems relating to the design thereof are disclosed and discussed at length in the article "Fan Clutches a Must for Heavy Trucks", Automotive Engineering Magazine, April 1975.
Another suggested solution to the fan drive problem centers around driving the fan with a hyraulic motor and controlling the hydraulic motor so that the fan is not driven when fan cooling is unnecessary, such as at high vehicle speeds. U.S. Pat. Nos. 3,650,567; 3,942,486; 3,217,697 and 2,769,394 disclose systems having hydraulic motors for driving a fan.
The present invention provides an improved fan drive system utilizing a hydraulic motor for driving the fan. In particular, the present invention provides a hydraulic system which includes a pilot-operated pressure-compensated control valve for controlling flow from a fluid source to the fan motor. The valve has a valve element for controlling flow between an inlet port, an outlet port in fluid communication with the fan motor, and a bypass port in fluid communication with a reservoir. The valve element is biased to a full open position to direct flow of fluid to the fan motor. The valve element is moved toward a closed position to divert flow to the reservoir in response to a pilot pressure signal. The pilot pressure signal is controlled by engine coolant temperature. The pilot pressure signal controls the valve element to divert flow to the reservoir when coolant temperature is below a predetermined minimum. Thus, when fan cooling of the engine is unnecessary, the fan motor is not driven.
The pilot-operated pressure-compensated control valve is designed to respond to pressure spikes or surges in the system to divert fluid from the outlet port to the bypass port. As a result of this construction, the valve provides a protection for the hydraulic motor. This constitutes a substantial improvement over prior art fan drive systems which have not included a pilot-operated valve having such functions.
Further, the pilot-operated pressure-compensated control valve is constructed so that it always allows at least a predetermined minimum flow from the source to the hydraulic fan motor. Thus, there is always a pressure maintained on the fan motor. The fan is therefore free to rotate due to the ram air impacting thereon and such rotation of the fan can occur without cavitation occurring in the hydraulic fan motor. Further, when it is desired to drive the fan by the hydraulic motor lower inertia forces need be overcome as compared to a system where the fan is stopped.
Accordingly, the present invention provides a pilot-operated pressure-compensated control valve which functions (1) to control the flow of fluid to the hydraulic fan motor in response to coolant temperature, (2) to respond to pressure spikes or surges in the system to divert fluid from the hydraulic fan motor, and (3) to maintain at least a minimum level of fluid flow from a fluid source to the hydraulic fan motor when the fan motor is not driving the fan.
As noted above, the pilot-operated pressure-compensated control valve has significant advantages when used to control flow to a fan motor. It may also be used in other fluid systems where the need arises.
Further, the present invention provides a combined steering and fan drive system in which fluid flow from a fixed displacement supply pump is directed to independent steering and fan motor drive systems under the control of a priority flow divider valve. The priority flow divider valve insures a predetermined amount of flow to the steering systems and excess flow to the fan drive system. Flow from the priority valve to the fan motor drive system is directed to the pilot operated pressure-compensated valve discussed above.